Experimental Study on Drillable Set Cement Capable of Strengthening Borehole Wall and Controlling Lost Circulation

Conventional compound lost circulation materials (LCMs) have no cementation among their particles. When these LCMs are put into the hole to stop mud losses, they are quite easy to be eroded away by the flow of drilling fluid. Cement slurry is another LCM frequently used to deal with severe mud losses, with a shortage of being easy to make new "hole" in a hole that uses cement slurry to stop mud losses. A Laboratory study has been conducted on drillable cement LCM. In this study, ultra-fine calcium carbonate was added into sulphoaluminate cement, a new gel material with adjustable drillability. It was found that when the concentration of ultrafine calcium carbonate in the cement slurry was not greater than 7%, the strength of the cement slurry was increasing with increase in the concentration of ultra-fine calcium carbonate. When the concentration of calcium carbonate was greater than 7%, the strength of the cement slurry was decreasing with increase in the concentration of ultra-fine calcium carbonate, i.e., the drillability of the cement was getting better. A sulphoaluminate cement treated with 0.4% water reducer, 0.3% viscosifier and 0.6% retarder had thickening time of 155 min, satisfying the needs of safe operation. A new lost circulation material was thus formulated by adding 12% scaly mica, 1% fiber and 1.5% limestone powders. This lost circulation material can be used to plug slots of 3 mm and 5 mm width, respectively. This drillable cement has provided a technical support for borehole wall strengthening and lost circulation control in drilling operations.